Method for non-intermittent provision of fluid supercool carbon dioxide at constant pressure above 40 bar as well as the system for implementation of the method

ABSTRACT

A method and an apparatus for the uninterrupted supply of liquid subcooled carbon dioxide. The liquid is supplied at a nearly constant pressure greater than about 40 bar. Liquid carbon dioxide is supplied at a low pressure and is sent into a low pressure tank where it is stored temporarily. The carbon dioxide is then pumped, with a pump, from the low pressure tank to a high pressure tank. During the pumping, the pressure of the carbon dioxide is increased. The carbon dioxide is stored in the high pressure tank until its removal. When the carbon dioxide is removed, it is in a thermodynamic disequilibrium between the liquid and gas phases.

BACKGROUND

The invention relates to a process and a supply system for theuninterrupted provision of liquid subcooled carbon dioxide at anessentially constant pressure greater than 40 bar.

In certain applications, large amounts of carbon dioxide at highpressure are required. An important aspect in this case is that thepressure is to be provided in as constant a manner as possible and theamount of carbon dioxide transported must be metered as accurately aspossible.

Recently carbon dioxide uses are being established, for example, whichrequire carbon dioxide at about 60 bar or above. For example, liquidcarbon dioxide at 60 bar is required for foaming plastics, insupercritical extraction, in chilling, in plasma spraying using laminarnozzles or in charging small carbon dioxide vessels.

In the production of polystyrene foam (XPS) by the mechanical blowingprocess, the blowing agent carbon dioxide used as an alternative isforced into the foam extruder at up to about 350 bar using a diaphragmmetering pump system. For the high pressure pumps, some manufacturersprescribe the use of room-temperature carbon dioxide which must bestored at a constant pressure and subcooled before entry into themetering pump.

To date, to provide liquid carbon dioxide at high pressure, a stationaryhigh-pressure tank has been filled with cold carbon dioxide at lowpressure (up to 20 bar). The carbon dioxide was then warmed, as a resultof which the pressure in the high-pressure tank increased to the desiredminimum pressure. During replenishment, the pressure had to be decreasedback to the low pressure level. The pressure was decreased by releasinggaseous carbon dioxide from the high-pressure tank, which gave rise tocosts and generally represented noise pollution for the environment.Furthermore, the supply with carbon dioxide was interrupted during thecharging period. In order to avoid interruption of the carbon dioxidesupply, two high-pressure tanks had to be mounted which were alternatelycharged and emptied. Not only the procurement costs of the twohigh-pressure vessels but also their maintenance costs due to theblow-off were considerable.

High-pressure storage in non-insulated heatable pressure vessels at 60bar and 22° C. is not able to continuously ensure high-pressureconditions. Since tanker trucks for industrial scale carbon dioxideconsumption always provide low-temperature low-pressure carbon dioxide(12 bar/−35° C.), the pressure in a high-pressure vessel collapsesduring replenishment. The supply pressure of the carbon dioxide must beelevated to the desired pressure level by an internal vessel heaterhaving an output-dependent time delay.

Charging high-pressure carbon dioxide vessels using the customary tankertruck pumps also posed problems, so that the pressure in the vessels hadto be released before charging to the maximum possible pump pressure.

Storage of low-temperature liquid carbon dioxide in a low-pressure tankand supplying a plant with liquid carbon dioxide at high pressure usinga pump has the disadvantage that in the event of pump faults, supply ofthe plant with carbon dioxide is interrupted and thus gives rise toconsiderable costs.

It was also disadvantageous with known processes that carbon dioxide wasalways provided in a state close to its boiling point. Liquids close totheir boiling point have a tendency to vapour formation, which makesmetering more difficult and makes transport relatively energy-intensiveowing to the compression losses which occur.

It is an object of the present invention, therefore, to specify animproved process and a supply system by which liquid carbon dioxide canbe provided uninterruptedly and inexpensively at an essentially constantpressure greater than 40 bar.

SUMMARY

This object is achieved according to the invention by a process and anapparatus as described herein. Advantageous embodiments and developmentseach of which can be employed individually or can be combined as desiredwith one another are subject matter of the respective dependent claims.

The inventive process for the uninterrupted provision of liquidsubcooled carbon dioxide at essentially constant pressure greater than40 bar comprises the following process steps:

-   -   the liquid carbon dioxide is supplied at low pressure;    -   the carbon dioxide is charged into a low-pressure tank and is        there stored temporarily;    -   the carbon dioxide is pumped by means of a pump from the        low-pressure tank into a high-pressure tank, the pressure of the        carbon dioxide being increased;    -   the carbon dioxide is stored or temporarily stored in the        high-pressure tank until removal in a thermodynamic        disequilibrium between a liquid phase and a gas phase.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects for the presentinvention, reference should be made to the following detaileddescription, taken in conjunction with the accompanying drawings, inwhich like elements are given the same or analogous reference numbersand wherein:

FIG. 1 illustrates diagrammatically an inventive supply system; and

FIG. 2 illustrates diagrammatically a piston pump used in the inventivesupply system according to the embodiment of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENTS

This object is achieved according to the invention by a process and anapparatus as described herein. Advantageous embodiments and developmentseach of which can be employed individually or can be combined as desiredwith one another are subject matter of the respective dependent claims.

The inventive process for the uninterrupted provision of liquidsubcooled carbon dioxide at essentially constant pressure greater than40 bar comprises the following process steps:

-   -   the liquid carbon dioxide is supplied at low pressure;    -   the carbon dioxide is charged into a low-pressure tank and is        there stored temporarily;    -   the carbon dioxide is pumped by means of a pump from the        low-pressure tank into a high-pressure tank, the pressure of the        carbon dioxide being increased; and    -   the carbon dioxide is stored or temporarily stored in the        high-pressure tank until removal in a thermodynamic        disequilibrium between a liquid phase and a gas phase.

The double temporary storage of the carbon dioxide permits uninterruptedprovision of carbon dioxide. If faults in the plant occur, in particularin the pump, the amount of carbon dioxide present in the high-pressuretank can be used for the supply until the plant is repaired. Thehigh-pressure tank has the function of a buffer reservoir.

Carbon dioxide in thermodynamic equilibrium begins to boil rapidly inthe case of small temperature decreases or temperature increases. Theintermediate storage of the carbon dioxide in thermodynamicdisequilibrium permits provision of subcooled carbon dioxide which doesnot exhibit this disadvantage in the known manner. The carbon dioxidedoes not form bubbles and is thus more easily transported and metered.Thermodynamic disequilibrium here means that the temperature of theliquid carbon dioxide is lower than the equilibrium temperature which isgiven by the prevailing pressure and the vapour-pressure curve. Thisthermodynamic disequilibrium occurs as a result of a nonhomogeneoustemperature distribution in the high-pressure tank, in particular asresult of a temperature gradient between the gaseous phase and theliquid phase of the carbon dioxide in the high-pressure tank. If thetemperature of the gaseous phase is higher than that of the liquidphase, a subcooled liquid is present.

The great advantage of the inventive process is that conditioned carbondioxide can be provided. In particular, the conditioned carbon dioxideis readily pumpable, does not have a tendency to (micro)bubbleformation, is present at a constant pressure and is provideduninterruptedly with great reliability. Costs of subsequent conditioningof the carbon dioxide are at least in part avoided. The operation ofsuch a process is comparatively inexpensive.

The high-pressure tank is designed in such a way that pressures between40 and 80 bar can be accepted. For this, the high-pressure tank isexpediently designed as a spherical vessel which has in particularthermal insulation, preferably a PU foam insulation, having a metaljacket of aluminium or galvanized steel. Since many applications requireliquid carbon dioxide at high pressure, the high-pressure tank exhibitsthe coexistence of a liquid phase and a gaseous phase of the carbondioxide. However, in principle, the high-pressure tank can also beoperated in the supercritical range, that is to say at above 73.7 bar.At pressures higher than 73.7 bar, the carbon dioxide is present inthermodynamic equilibrium in a single homogeneous phase which can beconsidered a high-density gas phase.

The low-pressure tank is designed for lower pressures, in particular forpressures less than 40 bar, in particular less than 30 bar, preferablyless than 25 bar. The low-pressure tank need not be designed as aspherical vessel and can be horizontal or vertical. Advantageously ithas a pressure-build-up device and a connection for carbon dioxide inthe liquid phase. The low-pressure tank has thermal insulation, inparticular vacuum insulation. The low-pressure tank can be charged fromconventional carbon dioxide tanker trucks. In the low-pressure tank aliquid phase and a gaseous phase of the carbon dioxide coexist inthermodynamic equilibrium.

By means of the pump the pressure of the carbon dioxide is increasedfrom the lower level of the low-pressure tank to the higher level of thehigh-pressure tank. As soon as the quantity or mass of carbon dioxide inthe high-pressure tank exceeds a preset value, liquid carbon dioxide ispumped from the low-pressure tank into the high-pressure tank. Thisensures that the high-pressure tank constantly has a sufficient amountof carbon dioxide, in particular two thirds, preferably three quarters,of a maximum capacity. This ensures that even with short-term faults ofthe system, in particular the pump, sufficient liquid carbon dioxide isstill present for supply. The pump ensures a pressure gradient betweenthe high-pressure tank and the low-pressure tank.

As a result of the double temporary storage of the carbon dioxide, thetemporary storage at a lower pressure level and the storage at a higherpressure level, uninterrupted provision of liquid carbon dioxide is madepossible. In particular, the carbon dioxide can be delivered at a lowpressure in a simple manner using a conventional tanker truck, withoutan interruption in the supply with carbon dioxide at high pressuretaking place.

In an embodiment of the inventive process, carbon dioxide from theliquid phase from the low-pressure tank is introduced into the liquidphase in the high-pressure tank to build up pressure in thehigh-pressure tank. By adding the liquid carbon dioxide directly to theliquid phase in the high-pressure tank the temperature of the gaseouscarbon dioxide in the high-pressure tank is essentially unchanged. Theincrease in the volume fraction of the liquid phase in the high-pressuretank caused by the addition produces a compression of the gaseous phasein the high-pressure tank, which increases the pressure in thehigh-pressure tank.

In a further embodiment of the inventive process, the liquid carbondioxide from the low-pressure tank is introduced into the gas phase inthe high-pressure tank to decrease the pressure in the high-pressuretank. As a result of adding the cold liquid carbon dioxide from thelow-pressure tank to the gaseous phase of the carbon dioxide in thehigh-pressure tank, a partial liquefaction of the gaseous carbon dioxidetakes place. As result the pressure in the high-pressure tank decreases.

Advantageously, the pressure of the carbon dioxide in the high-pressuretank is controlled by means of the fact that liquid carbon dioxide,depending on the current pressure in the high-pressure tank, is fedeither to the gas phase or the liquid phase in the high-pressure tank.Depending on whether the pressure in the high-pressure tank is too lowor too high, the pressure in the high-pressure tank can be kept constanteither by feeding liquid carbon dioxide directly to the liquid phase ofthe carbon dioxide in the high-pressure tank, or by adding liquid carbondioxide to the gaseous phase of the carbon dioxide, for example byspraying it into the gaseous phase.

In a further embodiment of the invention, the temperature of the liquidphase in the high-pressure tank is between 0 and 10° C., preferablybetween 2 and 5° C. These temperatures, at a pressure of around 60 bar,do not correspond to the temperature according to the equilibrium vapourpressure curve. The liquid is thus a subcooled liquid. The temperaturearises owing to a thermodynamic disequilibrium. This disequilibrium iscaused by a nonhomogeneous temperature distribution between liquid phaseand gas phase. Subcooled liquid carbon dioxide has the advantage that itdoes not have a tendency to vaporize and is readily pumpable.

Since many applications require liquid subcooled carbon dioxide, athermodynamic disequilibrium must be produced or maintained in thehigh-pressure tank. To produce or maintain the disequilibrium, accordingto the invention the liquid phase in the high-pressure tank is warmedlocally at one point, vaporized and/or converted into the gaseous phase.Expediently, the disequilibrium can be produced or maintained by localheating of gaseous carbon dioxide and/or by vaporizing liquid carbondioxide and/or by adding cold liquid carbon dioxide from thelow-pressure tank to the high-pressure tank. The local heating causes astabilization of the pressure in the high-pressure tank. Liquid carbondioxide is thus provided at a temperature which is lower than thatcorresponding to the vapour pressure curve.

Choosing an appropriate level of heating output in the local heatingcompensates for the loss of gaseous carbon dioxide owing to condensationof gaseous carbon dioxide. Also, proper choice of heating outputcompensates for the pressure drop in the high-pressure tank owing totake-off of liquid carbon dioxide.

For further pressure stabilization and to ensure a minimum pressure inthe high-pressure tank, in particular during replenishment with coldcarbon dioxide from the low-pressure tank, the liquid phase and/or thegas phase in the high-pressure tank is warmed. The warming is performed,in particular, by separate heating systems.

If, for example, cold carbon dioxide from the low-pressure tank is fedto the high-pressure tank via the gas phase, the temperature of theliquid carbon dioxide in the high-pressure tank falls. As a result,gaseous carbon dioxide condenses in the high-pressure tank. Thetemperature decrease produces a fall in pressure in accordance with thevapour-pressure curve. To avoid such pressure fluctuations duringcharging, the liquid cold carbon dioxide fed is passed in a definedratio both into the gas phase and the liquid phase of the high-pressuretank.

An excessive fall in temperature of the liquid phase in thehigh-pressure tank due to adding cold carbon dioxide from thelow-pressure tank is prevented by a second heater. By means of thesecond heater, the subcooling of the carbon dioxide towards lowtemperatures is limited.

Advantageously, the carbon dioxide is fed from the low-pressure tank tothe high-pressure tank as soon as the volume or mass of carbon dioxidein the high-pressure tank falls below a preset value. A suitable controlcircuit ensures by this means that sufficient liquid carbon dioxide isalways present in the high-pressure tank. In particular in the event ofpump faults or temporary restrictions in supplying the high-pressuretank with liquid carbon dioxide, this buffer ensures a safety periodwhich can be utilized for remedying the fault. For example, thehigh-pressure tank is filled with liquid carbon dioxide as soon as thehigh-pressure tank is less than three-quarters full. In the event of afault, thus at least the volume of a three-quarters-full high-pressuretank is available. This measure considerably increases the security ofsupply.

In one embodiment of the invention, the low pressure is less than 40bar, in particular less than 30 bar, preferably less than 25 bar. At lowpressures, transport using conventional tanker trucks is simpler andcheaper.

Advantageously, to ensure a minimum pressure in the low-pressure tank,the liquid carbon dioxide in the low-pressure tank is warmed. This alsoprevents solid carbon dioxide (dry ice) from forming in the low-pressuretank. In particular, when the pump withdraws relatively large amounts ofcarbon dioxide from the low-pressure tank and feeds them to thehigh-pressure tank, the pressure in the low-pressure tank decreases ifinsufficient liquid carbon dioxide vaporizes and passes over into thegas phase for pressure compensation.

When low-temperature carbon dioxide is fed to the low-pressure tank froma tanker truck, the pressure in the low-pressure tank also usuallydecreases, since with the addition of colder carbon dioxide thetemperature in the low-pressure tank falls and the pressure follows thedrop in temperature in accordance with the vapour-pressure curve.Heating the carbon dioxide causes a temperature elevation, by whichmeans a pressure drop can be compensated for.

In one embodiment of the invention, to charge the pump with bubble-freecarbon dioxide, the gaseous carbon dioxide formed in the first lineand/or in the pump is recirculated to the low-pressure tank. Theefficiency of the pump is thereby increased, since this avoidsunnecessary compression of gaseous carbon dioxide.

The inventive supply system for uninterrupted provision of subcooledcarbon dioxide at an essentially constant pressure greater than 40 barcomprises a low-pressure tank and a high-pressure tank, each for holdinga liquid phase and a gas phase, and a pump, in which case the pump isdisposed between the low-pressure tank and the high-pressure tank and isconnected by a first line to the low-pressure tank and the pump isconnected by a second line to the high-pressure tank. Advantageously,the second line transforms into an upper and lower feed line, the upperfeed line opening out into an upper region of the high-pressure tank,and the lower feed line opening into a lower region of the high-pressurefeed tank.

Via the first line, the pump and the upper or lower feed line, thelow-pressure tank and the high-pressure tank are connected to oneanother. The pump produces the pressure difference between the pressurelevels in the two tanks.

Liquid carbon dioxide is fed from the low-pressure tank to thehigh-pressure tank from the top via the upper feed line. Liquid carbondioxide thus falls through the gas phase in the high-pressure tank, asresult of which gaseous carbon dioxide is condensed. This causes thepressure to fall in the high-pressure tank.

Liquid carbon dioxide is fed from the low-pressure tank via the lowerfeed line to the liquid carbon dioxide in the high-pressure tank. As aresult the volume of the liquid phase in the high-pressure tankincreases, whereby the gaseous phase is compressed. This causes thepressure in the high-pressure tank to increase.

In a particular embodiment of the inventive supply system, thehigh-pressure tank has a first heater which is disposed in an additionalline on the high-pressure tank, which line joins a lower region of thehigh-pressure tank for the liquid phase to a higher region of thehigh-pressure tank for the gas phase.

Using the first heater, liquid carbon dioxide is vaporized locally atone point to produce a minimum pressure in the high-pressure tank. Athermodynamic disequilibrium is hereby produced or maintained. The localheating of carbon dioxide at one point, with the thermodynamicdisequilibrium being maintained, compensates for the rate ofcondensation of the carbon dioxide condensing from the gas phase by therate of vaporization of the carbon dioxide passing from the liquid phaseto the gaseous phase.

By means of the interaction of the warming by the first heater and thecooling by an addition of cold carbon dioxide from the low-pressuretank, subcooled liquid carbon dioxide is provided by the high-pressuretank at a high pressure and presettable temperature. This saves, atleast in part, considerable costs for conditioning the carbon dioxide.

The upper feed line advantageously opens into an upper region of thehigh-pressure tank. If the liquid carbon dioxide is passed from thelow-pressure tank to the high-pressure tank through the upper region ofthe high-pressure tank containing the gas phase, the temperaturedistribution in the high-pressure tank becomes homogeneous. Thehomogeneity of the temperature distribution can in turn be altered bytargeted local heating of the gaseous and/or the liquid phase. Theinteraction between homogeneity and nonhomogeneity is used, in thecontext of control, for providing conditioned, that is to say liquid andsubcooled, carbon dioxide at a constantly high pressure.

By controlling the timely supply of the high-pressure tank with carbondioxide from the low-pressure tank, the security of supply isconsiderably increased. Even technical faults of the pump do notinevitably lead to an interruption in supply with carbon dioxide, sincea large amount of liquid carbon dioxide is present to maintain thecarbon dioxide supply during the time of repair or replacement of thepump.

For further support of a minimum pressure in the high-pressure tank, andalso to ensure a minimum temperature in the high-pressure tank, thehigh-pressure tank has a second heater which is disposed in the lowerregion of the high-pressure tank. If, for example, the temperature ofthe liquid carbon dioxide in the high-pressure tank falls below a presetvalue owing to the addition of cold carbon dioxide from the low-pressuretank, the temperature can be increased by the second heater. Using thesecond heater, a temperature difference between the liquid and gaseousphases in the high-pressure tank can be levelled out.

Since the low-pressure tank has a low pressure less than 40 bar, inparticular less than 30 bar, preferably less than 25 bar, the lowpressure tank can be charged by conventional tanker trucks for carbondioxide. In order that the low-pressure tank can store cold carbondioxide, in particular carbon dioxide at less than −10° C., thelow-pressure tank has thermal insulation. In a special embodiment of theinvention, the low-pressure tank has a pressure build-up device, bywhich means the pressure in the low-pressure tank can be built up.

The high-pressure tank is constructed in such a manner that it canaccept pressures which are required by the respective application. Thehigh-pressure tank can withstand pressures of at least 40 bar, inparticular at least 50 bar, preferably at least 60 bar. In order thatthe high-pressure tank can hold subcooled liquid carbon dioxide, thehigh-pressure tank is expediently thermally insulated.

To counteract a general warming of the carbon dioxide in thelow-pressure tank, the low-pressure tank has a cooler. This preventsexcessive pressure increase in the low-pressure tank.

A minimum temperature in the low-pressure tank, in particular whenlow-temperature carbon dioxide is added from a tanker truck, is ensuredby heating by means of a further heater for the liquid carbon dioxidephase. Even in the event of high takeoff of liquid carbon dioxide fromthe low-pressure tank by the high-pressure tank, by heating using thisheater, sufficient liquid carbon dioxide is vaporized and converted intothe gas phase to counteract a pressure drop in the low-pressure tank.

In order to transport the carbon dioxide from the low-pressure tank tothe high-pressure tank efficiently, the low-pressure tank has aconnection for the liquid phase for the first line. Large amounts ofcarbon dioxide may be transported better using a pump with a compressor,since a compressor to a great degree only performs work on the gas,which increases the internal energy of the gas. This portion of the workexpended is lost as heat and is not used for the actual pumping of thecarbon dioxide.

In a special embodiment, a return line is provided between the secondline and the low-pressure tank, by means of which return line gaseouscarbon dioxide can be recirculated to the low-pressure tank. This isimportant in particular when turning on the pump, if much gaseous carbondioxide is formed during cooling of the pumps.

For open-loop or closed-loop control of the supply system, aninstrumentation system having sensors is provided that determines atleast one parameter selected from the group consisting of quantity ofcarbon dioxide or mass of carbon dioxide in the high-pressure tank,quantity of carbon dioxide or mass of carbon dioxide in the low-pressuretank, pressure in the high-pressure tank, pressure in the low-pressuretank, temperature of the liquid phase in the high-pressure tank,temperature of the carbon dioxide in the low-pressure tank andtemperature of the pump.

Determining the carbon quantity in the high-pressure tank., for exampleby carbon dioxide mass determination establishes when replenishment ofthe high-pressure tank by carbon dioxide from the low-pressure tankusing the pump is necessary.

By determining the carbon dioxide quantity or carbon dioxide mass in thelow-pressure tank, delivery dates are established for new carbon dioxidefrom a tanker truck.

The pressure in the high-pressure tank and in the low-pressure tank ismeasured in order to, firstly, prevent excessive overpressure in thehigh-pressure tank, and secondly to recognize faults in the operation ofthe supply system. In particular for applications which necessitate aparticularly constant high pressure, pressure monitoring in thehigh-pressure tank is required.

With the aid of measuring the temperature of the liquid carbon dioxidein the high-pressure tank, a minimum temperature required for manyapplications is ensured. If the temperature falls below a preset value,heating is performed. Temperature measurement is also necessary in orderto ensure that a maximum temperature of the carbon dioxide in thehigh-pressure is not exceeded.

Measuring the temperature of the carbon dioxide in the low-pressure tankand of the pump is expedient for checking the status of the supplysystem.

Advantageously, the supply system comprises a control unit which isconnected to the instrumentation system and at least one componentselected from the group consisting of pump, second heater for the liquidphase in the high-pressure tank, first heater for the liquid phase inthe high-pressure tank, cooler in the low-pressure tank, first valve inthe first line, second valve in the second line, third valve in thesecond line, return line valve in the return line between the secondline and the low-pressure tank, first safety valve on the low-pressuretank and second safety valve on the high-pressure tank.

By means of the control unit and the pump, a sufficient liquid level inthe high-pressure tank, for example, is ensured.

By means of the second heater for liquid carbon dioxide in thehigh-pressure tank, a minimum temperature of the liquid carbon dioxidein the high-pressure tank is ensured.

Using the first heater, liquid carbon dioxide is vaporized locally atone point in the high-pressure tank, which builds up and maintains athermodynamic disequilibrium in the high-pressure tank.

Controlling the cooling ensures that a maximum temperature, and thus amaximum pressure, in the low-pressure tank is not exceeded.

Using the first valve, at times when the pump is not required, the pumpcan be decoupled from the low-pressure tank, so that stressing the pumpwith low temperatures is avoided.

Using the second valve, for the period when the pump is not inoperation, the pump is decoupled from the high-pressure tank.

Using the third valve in the second line, the cold liquid carbon-dioxidestream is either passed directly into the liquid carbon dioxide in thehigh-pressure tank, whereby the pressure in the high-pressure tank isincreased, or is passed into the gas phase of the high-pressure tank,whereby the pressure is reduced.

By means of the return line valve in the return line between the secondline and the low-pressure tank, gaseous carbon dioxide can berecirculated in a controlled manner into the low-pressure tank. This isimportant, in particular, when, on turning on the pump, liquid carbondioxide is vaporized during cooling of the pump. Pumping gaseous carbondioxide is energy-consuming and endangers the functionality of thehigh-pressure pump.

Controlling the first safety valve on the low-pressure tank and thesecond safety valve on the high-pressure tank prevents the low-pressuretank or the high-pressure tank from being excessively loaded.

In an advantageous embodiment of the inventive supply system, to takeoff the carbon dioxide from the liquid phase, the high-pressure tank hasa dewatering valve and/or a descender tube. By means of the dewateringvalve and/or the descender tube, the liquid phase of the carbon dioxideis taken off from the high-pressure tank in a simple manner.

Advantageously, the pump is a piston pump having a displacement space,in particular a three-piston pump, which is arranged and/or constructedin such a manner that gas cannot collect in the suction space duringoperation. Thus, gas collection in the displacement space is largelyprevented.

Collections of gas in the displacement space lead to high energy losses,since the work applied by the pump is not used for pumping the liquidcarbon dioxide, but for compressing the gaseous phase of the carbondioxide. This leads only to increasing the internal energy of the carbondioxide, in particular to elevating its temperature, and isenergy-consuming.

By means of a suitable arrangement of the control valves, thedisplacement space of the piston pump is always filled with liquidcarbon dioxide. Gaseous carbon dioxide can escape from the suctionspace; collection of gaseous carbon dioxide is avoided.

Additional degassing orifices or channels which lead off gaseous carbondioxide from the displacement space, in particular to the low-pressuretank, are expedient in order to ensure that the displacement space isalways filled solely with liquid carbon dioxide.

Advantageously, to remove the gaseous phase from the suction space, atakeoff line is present between an inlet of a pump and an upper part ofthe low-pressure tank. Gaseous carbon dioxide thus escapes from thesuction space of the piston pump and passes via the takeoff line to thelow-pressure tank.

In a special embodiment of the inventive supply system, thehigh-pressure tank has a capacity of less than 2 t, in particular lessthan 1.5 t, preferably less than 1.2 t, of carbon dioxide.

Compared with high-pressure tanks which are customary for industrialscale applications, a high-pressure tank of the inventive supply systemis small. Such small high-pressure tanks are inexpensive and, owing tothe interaction between low-pressure tank and high-pressure tank, arecompletely sufficient to provide an uninterrupted continuous flow ofcarbon dioxide in large quantities.

The low-pressure tank advantageously has a capacity of at least 3 t, inparticular at least 7 t, preferably at least 10 t, of carbon dioxide. Asa result of such a large dimensioning of the low-pressure tank, asufficiently large quantity of carbon dioxide is stored temporarily fora high carbon dioxide consumption in corresponding industrial scaleapplications, so that the supply system is comparatively independent ofshort-term supply restrictions during delivery of carbon dioxide fromtanker trucks.

Further advantageous embodiments are described with reference to thedrawing below. The drawing is not intended to restrict the scope of theinvention, but only to illustrate this by way of examples.

In the drawing:

FIG. 1 shows diagrammatically an inventive supply system and

FIG. 2 shows diagrammatically a piston pump used in the inventive supplysystem according to FIG. 1.

FIG. 1 shows an inventive supply system 3 having a low-pressure tank 1and a high-pressure tank 2 in which in each case liquid and gaseouscarbon dioxide are present as coexisting phases. The low-pressure tank 1is connected via a first line 5 to a pump 4 and, via a second line 6 oran upper feed line 40 and a lower feed line 41, from the pump 4 to thehigh-pressure tank 2.

By means of a first valve 25 in the first line 5 and a second valve 26in the second line 6, the pump 4 can be decoupled from the low-pressuretank 1 and the high-pressure tank 2 when the pump 4 is not in operationor must be serviced. Via an inlet tube 36 having an inlet valve 37, thelow-pressure tank 1 is charged from a tanker truck with cold liquidcarbon dioxide at −35° C. and 15 bar.

To restrict the pressure in the low-pressure tank, the carbon dioxide isstabilized in temperature by an insulation 7, in that the insulation 7decreases heat flux from the outside to the carbon dioxide in thelow-pressure tank. The cooler 10 has the task of counteracting a warmingof the carbon dioxide due to a heat flux from the outside. A safetyvalve 23 ensures that in the event of excessive temperature increase amaximum permissible maximum pressure is not exceeded. If the pressurereaches this maximum pressure, gaseous carbon dioxide is discharged, asa result of which the temperature of the liquid carbon dioxide fallsowing to the heat of evaporation of the liquid carbon dioxide.

The pump 4 takes off liquid carbon dioxide from the low-pressure tank 1at a liquid port 13. If so much liquid carbon dioxide is taken off fromthe low-pressure tank 1 that the pressure in the low-pressure tank 1falls excessively, which would cause a decrease in temperature of thecarbon dioxide in the low-pressure tank 1, or if too much cold liquidcarbon dioxide is charged into the low-pressure tank, the liquid phasein the low-pressure tank 1 is heated.

The pump 4 is constructed as a piston pump and has an inlet 21 which isjoined to the low-pressure tank 1 via a return line 27 in which isdisposed a return valve 28. By means of the return line 27, gaseouscarbon dioxide which has formed either in the first line 5 or in thepump 4 is passed back to the low-pressure tank 1, so that the pump 4 ischarged solely with liquid carbon dioxide and not also with gaseouscarbon dioxide. By means of a return line 14 which has a return valve15, during a cold start-up phase, liquid and/or gaseous carbon dioxidein the second line 6 is recirculated to the low-pressure tank 1 when thesecond valve 26 is closed. These measures prevent a considerable part ofthe work performed by the pump 4 from being lost by compression of thegaseous phase of the carbon dioxide being performed as a significantpart of the work only to increase the internal energy of the carbondioxide.

The high-pressure tank 2 has an upper region 11 for the gaseous phase ofthe carbon dioxide and a lower region 12 for the liquid phase of thecarbon dioxide. The upper feed line 40 opens into the upper region 11 ofthe high-pressure tank 2. The lower feed line 41 opens into the lowerregion 12. Depending on the current pressure, a third valve 42 and afourth valve pass the carbon dioxide stream into the high-pressure tank2 via the upper feed line 40 or lower feed line 41. If carbon dioxide isfed via the upper feed line 40, the gas phase cools and the pressure inthe high-pressure vessel decreases. If carbon dioxide is fed via thelower feed line 41, the gas phase above the liquid phase is compressedand the pressure in the high-pressure vessel increases.

As a result of addition of liquid carbon dioxide from the low-pressuretank 1, the temperature in the high-pressure tank 2 falls. Thehigh-pressure tank 2 contains a first heater 29 for local heating andvaporization of liquid carbon dioxide in order to build up and maintaina thermodynamic disequilibrium.

By means of the different ways of feeding with the upper feed line 40and lower feed line 41, and by means of the first heater 29, thesubcooled state of the carbon dioxide is produced and maintained.

The high-pressure tank 2 has a second heater 9 for heating the liquidphase, which can be used to set a minimum temperature of the carbondioxide.

If liquid carbon dioxide is taken off from the high-pressure tank 2 viaa takeoff point 20 which has a dewatering valve 16, the pressure in thehigh-pressure tank 2 first decreases.

Using the first heater 29, liquid carbon dioxide can be converted intothe gaseous phase, so that a thermodynamic disequilibrium is maintainedin the high-pressure tank 2 at a constant pressure.

Subcooled liquid carbon dioxide is provided by means of the fact thatthe gaseous phase of the carbon dioxide is not in thermodynamicequilibrium with the liquid phase and the two phases have differenttemperatures.

However, on account of the vapour-pressure curve, a temperaturedifference leads to vaporization or condensation of carbon dioxide atthe phase boundary. Especially in the case of subcooled carbon dioxidethis leads to gaseous carbon dioxide condensing at the phase boundaryand transferring to the liquid phase. This condensation and theassociated loss of carbon dioxide in the gaseous phase leads to apressure drop in the low-pressure tank 2 if sufficient liquid carbondioxide is not fed to the gaseous phase via an additional line 30 forcompensation using the first heater 29. Via choice of the heating outputlevel of the first heater 29, a pressure drop in the high-pressure tank2 can be prevented.

The second heater 9 has the task of ensuring a preset minimumtemperature of the liquid phase in the high-pressure tank 2.

The heaters 9, 29 and the cooler 10 are connected by a control unit 18.The control unit 18 controls the heaters 9, 29, the cooler 10 and thepump 4 as a function of the data determined by an instrumentation system17, for example the pressures, temperatures and liquid levels in thesupply system 3.

A general warming of the carbon dioxide in the high-pressure tank 2counteracts cooling as a result of the addition of cold carbon dioxidefrom the low-pressure tank 1. By suitable choice of the heater outputlevels in the high-pressure tank 2, and the carbon dioxide feed to thehigh-pressure tank 2, subcooled carbon dioxide is provideduninterruptedly at a constant pressure of about 60 bar.

A safety valve 24 protects the high-pressure tank 2 from an excessiveoverpressure.

The liquid carbon dioxide from the high-pressure tank can be taken offeither via the takeoff point 20 or via a descender tube.

FIG. 2 shows a pump 4 used in the inventive supply system 3 having adrive 32 and a displacement space 31.

The suction valve is arranged in such a manner that only liquid carbondioxide passes into the displacement space and as a result energy lossesdue to compression of gaseous carbon dioxide are avoided.

The inventive process for the uninterrupted provision of liquidsubcooled carbon dioxide at essentially constant pressure greater than40 bar comprises the following process steps: liquid carbon dioxide isdelivered at a low pressure, the carbon dioxide is charged into alow-pressure tank 1 and stored there temporarily; the carbon dioxide ispumped from the low-pressure tank 1 to a high-pressure tank 2, thepressure of the carbon dioxide being increased and the carbon dioxide isstored temporarily in the high-pressure tank 2 in a thermodynamicdisequilibrium until takeoff.

The process and the supply system 3 suitable for carrying out theprocess are distinguished by their high performance and efficiency forthe uninterrupted and inexpensive supply of liquid subcooled carbondioxide at essentially constant pressure greater than 40 bar.

It will be understood that many additional changes in the details,materials, steps and arrangement of parts, which have been hereindescribed in order to explain the nature of the invention, may be madeby those skilled in the art within the principle and scope of theinvention as expressed in the appended claims. Thus, the presentinvention is not intended to be limited to the specific embodiments inthe examples given above.

LIST OF DESIGNATIONS

-   1 Low-pressure tank-   2 High-pressure tank-   3 Supply system-   4 Pump-   5 First line-   6 Second line-   7 Insulation-   9 Second heater-   10 Cooler-   11 Upper region-   12 Lower region-   13 Liquid port-   14 Return line-   15 Return line valve-   16 Dewatering valve-   17 Instrumentation system-   18 Control unit-   19 Gas displacement line-   20 Takeoff point-   21 Inlet-   23 Safety valve-   24 Safety valve-   25 First valve-   26 Second valve-   27 Return line-   28 Return line valve-   29 First heater-   30 Additional line-   31 Displacement space-   32 Drive-   33 Piston-   34 First valve-   35 Support-   36 Intake tube-   37 Intake valve-   38 Housing-   39 Second valve-   40 upper feed line-   41 lower feed line-   42 third valve-   43 suction space

1. A method which may be used for the uninterrupted supply of liquid subcooled carbon dioxide at a nearly constant pressure of greater than about 40 bar, the method comprising: a) supplying liquid carbon dioxide at low pressure; b) introducing said low pressure liquid into a low pressure tank for temporary storage, said low pressure being less than 40 bar; c) pumping said low pressure liquid, with a pump means, from said low pressure tank into a high pressure tank, wherein the pressure of said liquid is increased by said pumping; d) storing said increased pressure liquid in said high pressure tank, wherein said increased pressure liquid is pumped into an upper region of said high pressure tank via an upper feed line or a lower region of said high pressure tank via a lower feed line depending upon a pressure in said high pressure tank; and e) removing said increased pressure liquid from said high pressure tank in a state of thermodynamic disequilibrium between the liquid and gas phases, the liquid phase in said high pressure tank is between about 0° C. and about 10° C.
 2. The method of claim 1, wherein pressure is increased in said high pressure tank by adding said low pressure liquid from said low pressure tank to the liquid phase in said lower region of said high pressure tank.
 3. The method of claim 1, wherein pressure is decreased in said high pressure tank by adding said low pressure liquid from said low pressure tank to the gas phase in said upper region of said high pressure tank.
 4. The method of claim 1, further comprising controlling the pressure in said high pressure tank by adding said increased pressure liquid to at least one member selected from the group consisting of: a) the gas phase in said high pressure tank; and b) the liquid phase in said high pressure tank.
 5. The method of claim 1, wherein said temperature of said liquid phase in said high pressure tank is between about 2° C. and about 5° C.
 6. The method of claim 1, further comprising producing said thermodynamic disequilibrium in said high pressure tank by locally warming the liquid phase in said high pressure tank to convert said liquid phase into the gas phase.
 7. The method of claim 6, further comprising maintaining said thermodynamic disequilibrium in said high pressure tank by locally warming said liquid phase in said high pressure tank to convert said liquid phase into said gas phase.
 8. The method of claim 1, further comprising stabilizing the pressure in said high pressure tank, wherein said stabilizing comprises warming at least one member selected from the group consisting of: a) the liquid phase of said high pressure tank; and b) the gas phase of said high pressure tank.
 9. The method of claim 8, wherein each said warming is performed by a separate heating system.
 10. The method of claim 1, wherein said low pressure is less than about 30 bar.
 11. The method of claim 10, wherein said low pressure is less than about 25 bar.
 12. The method of claim 1, wherein additional low pressure liquid is pumped into said high pressure tank as soon as the quantity or mass of carbon dioxide in the high pressure tank exceeds a preset value.
 13. A method which may be used for the uninterrupted supply of liquid subcooled carbon dioxide comprising: a) supplying liquid carbon dioxide at low pressure; b) introducing said low pressure liquid into a low pressure tank for temporary storage; c) pumping said low pressure liquid, with a pump means, from said low pressure tank into a high pressure tank, wherein the pressure of said liquid is increased by said pumping; d) storing said increased pressure liquid in said high pressure tank wherein said increased pressure liquid is pumped into an upper region of said high pressure tank via an upper feed line or a lower region of said high pressure tank via a lower feed line depending upon a pressure in said high pressure tank; e) removing said increased pressure liquid from said high pressure tank in a state of thermodynamic disequilibrium between the liquid and gas phases; f) feeding said increased pressure liquid from said low pressure tank to said high pressure tank when the mass of said carbon dioxide in said high pressure tank is less than about one quarter of a high pressure tank maximum capacity.
 14. The method of claim 13, wherein said increased pressure liquid is fed to said high pressure tank when said mass is less than about one third of said capacity.
 15. The method of claim 13, wherein said low pressure is less than about 40 bar.
 16. A method which may be used for the uninterrupted supply of liquid subcooled carbon dioxide at a constant pressure greater than about 40 bar comprising: a) supplying liquid carbon dioxide at low pressure; b) introducing said low pressure liquid into a low pressure tank for temporary storage, said low pressure being less than 40 bar; c) pumping said low pressure liquid, with a pump means, from said low pressure tank into a high pressure tank, wherein the pressure of said low pressure liquid is increased by said pumping; d) storing said increased pressure liquid in said high pressure tank wherein said increased pressure liquid is pumped into an upper region of said high pressure tank via an upper feed line or a lower region of said high pressure tank via a lower feed line depending upon a pressure in said high pressure tank; e) removing said increased pressure liquid from said high pressure tank in a state of thermodynamic disequilibrium between the liquid and gas phases; and f) providing said pump means with bubble-free liquid by recirculating any gaseous carbon dioxide, found in the line from said low pressure tank to said means, back to said low pressure tank.
 17. A system used for the uninterrupted provision of subcooled carbon dioxide at a constant pressure greater than about 40 bar, comprising: a) a low pressure tank containing carbon dioxide in liquid and gas phases; b) a high pressure tank containing carbon dioxide in liquid and gas phases; c) a pump located between said low pressure tank and said high pressure tank; d) a first line connecting said low pressure tank to said pump; e) a second line connecting said high pressure tank to said pump; f) a second line valve in fluid communication between and with said second line and said high pressure tank; g) an upper feed line in fluid communication between and with said second line valve and an upper region of said high pressure tank; h) a lower feed line in fluid communication between and with said second line valve and a lower region of said high pressure tank; i) a third line connecting a lower region of said high pressure tank with an upper region of said high pressure tank; and j) a first heater located on said third line.
 18. A method which may be used for the uninterrupted supply of liquid subcooled carbon dioxide at essentially constant pressure greater than 40 bar comprising: a) supplying liquid carbon dioxide at low pressure; b) introducing said low pressure liquid into a low pressure tank for temporary storage, said low pressure being less than 40 bar; c) pumping said low pressure liquid, with a pump means, from said low pressure tank into a high pressure tank, wherein the pressure of said liquid is increased by said pumping; d) storing said increased pressure liquid in said high pressure tank and wherein said liquid is pumped into an upper region of said high pressure tank via an upper feed line or a lower region of said high pressure tank via a lower feed line depending upon a pressure in said high pressure tank; e) removing said increased pressure liquid from said high pressure tank in a state of thermodynamic disequilibrium between the liquid and gas phases, wherein the temperature of the liquid phase in said high pressure tank is between about 0° C. and about 10° C.; and f) stabilizing the pressure in said high pressure tank by warming at least one member selected from the group consisting of the liquid phase of said high pressure tank and the gas phase of said high pressure tank.
 19. A method which may be used for the uninterrupted supply of liquid subcooled carbon dioxide at a nearly constant pressure of greater than about 40 bar, the method comprising: a) supplying liquid carbon dioxide at low pressure; b) introducing said low pressure liquid into a low pressure tank for temporary storage, said low pressure being less than 40 bar; c) pumping said low pressure liquid, with a pump means, from said low pressure tank into a high pressure tank, wherein the pressure of said liquid is increased by said pumping; d) storing said increased pressure liquid in said high pressure tank, wherein said increased pressure liquid is pumped into an upper region of said high pressure tank via an upper feed line or a lower region of said high pressure tank via a lower feed line depending upon a pressure in said high pressure tank; e) removing said increased pressure liquid from said high pressure tank in a state of thermodynamic disequilibrium between the liquid and gas phases; and f) stabilizing the pressure in said high pressure tank by warming at least one member selected from the group consisting of the liquid phase of said high pressure tank or the gas phase of said high pressure tank. 